NMR spectroscopy is a method of instrumental analysis, which can determine the chemical composition of samples. Radio frequency (RF) pulses are emitted into the sample, which is located in a strong, homogeneous, static magnetic field B0, and the electromagnetic reaction of the probe is measured. With solid state NMR spectroscopy, the technique of magic angle spinning (MAS) is used to reduce line broadening due to anisotropic interactions of the sample. In NMR-MAS the sample is tilted to the so-called “magic angle” of θm=arccos(√⅓)≈54.74° with respect to the homogenous, static magnetic field. The angle θm is a solution of the second order of Legendre polynomials P2(cos(θm))=0, so that all interactions dependent on this Legendre polynomial disappear at this angle to the magnetic field. This is the case for three important interactions in solids: dipolar coupling, chemical shift anisotropy and quadrupole interaction of the first order. For polycrystalline measurement samples, the crystal directions of the individual crystals are randomly oriented with respect to the static field The elimination of the interactions is achieved by a sufficiently fast rotation of the measurement sample at the magic angle. In this way, line broadening due to these interactions can be significantly reduced, ideally even to the natural line width.
NMR-MAS probeheads allow high resolution NMR spectroscopy to be carried out with solid, powder, or semi-solid (e.g., gel or paste) measurement samples. The measurement sample is poured into a circular cylinder sample holder, also called the rotor, which is rotated at very high speeds (e.g., with a rotation frequency in the range of a few kHz to over a hundred kHz) via compressed gases in a stator. The radial position is secured by air bearings in the stator, while the holding force created by the air flow holds the rotor in its axial position in the stator.
In general these probeheads are used in superconducting NMR magnet systems, in which the homogeneous static magnetic field B0 is oriented along a “bore hole,” which specifies the z-axis of the laboratory coordinate system. Such a magnet system is illustrated in FIG. 5. In general, the magnet system comprises a magnet and at least one shim system. The magnet normally includes a dewar/vacuum isolation, radiation shields, at least one magnet coil and safety elements. In general, the magnet coil is laid out in such a way that it is actively shielded to/from the outside environment, in order to minimize any influence of susceptibility changes in the direct environment of the magnet on its field. The shim system is frequently designed in two parts, and includes a cryogenically cooled part made of superconductive wire inside the dewar, and a room temperature shim system, which is arranged inside the bore hole of the dewar. The magnet system includes a cavity into which the NMR probehead is inserted during a measurement. In general, this cavity is substantially tubular in sections, but it can also contain conical sections, which may serve as centering devices for probeheads, samples etc.
In FIGS. 6A and 6B, the sample 5, the rotation axis RA and the walls of the cavity 6 are shown in two orthogonal sections. Further elements of the NMR probehead, such as RF coils, stator, walls, networks etc., are not shown for the sake of simplicity. The rotation axis RA of the sample is also described as the z′-axis, and has a joint origin with the z-axis. The z- and z′-axes lie on a plane defined by the x- and z-axes, as well as the x′- and z′-axes. The y- and y′-axes of the two coordinate systems are identical.
For many NMR experiments in magnet systems with B0 fields in the range from 7 T to 25 T, an adjustment precision of the magic angle from 0.1° to 0.01° is sufficient. However, for some applications (e.g., satellite transition (ST-MAS) NMR) a precision of up to 0.001° is required. The angle adjustment should remain constant over a wide range of temperatures, and be reproducible when changing samples.
In the prior art, with most MAS probeheads, the angle adjustment is carried out by a mechanism integrated into the probehead (e.g., as described in U.S. Pat. No. 7,498,812 B2, US 2014/0099730 A1, U.S. Pat. Nos. 7,535,224 B2, or 5,260,657), as shown schematically in FIG. 8. Various methods such as using hoists with end stops, rods and levers, gear wheels, spindles, etc., are common. These methods are also in use for hermetically sealed probeheads, where the temperatures of sample and detection coil may differ significantly, and are carried out with bellows or O-ring seals (e.g., as described in U.S. Pat. No. 7,282,919 B2) for the sealed probeheads.
The extremely high requirements for the precision and reproducibility of the mechanism present a technological challenge to integrating the angle adjustment into the probehead. Assuming typical lever lengths in the range from approximately 2 cm to 3 cm, the required angular precision results in a mechanical tolerance of approximately 0.5 μm to 5 μm. Such narrow tolerances lead to high manufacturing costs for the mechanical components.
MAS probeheads typically cover a very wide temperature range for the samples. Probeheads designed for the lower end of the temperature scale may be intended specifically for temperatures down to −50° C., −80° C., −130° C., or temperatures in the cryogenic realm from 30 K to 100 K. Probeheads may also be designed for the upper limit values of temperature control (e.g., temperatures up to +80° C., +150° C., or far higher in the case of special samples). The temperature control of the samples is ensured by a temperature control gas in most cases, which may also control the temperature of the bearing air and/or the drive air.
Due to the compactness of the construction (the sample diameters are typically in the range from 0.7 mm to 4 mm), the temperature of at least part of the tilt mechanism is close to the temperature of the samples. The reproducibility of the adjustment of an angle with high precision and over a wide range of temperatures, is technically extremely difficult to implement, and leads to high costs in the manufacturing of the mechanical parts.
Furthermore, an NMR system may include a probehead without internal adjustment (e.g., as described in U.S. Pat. No. 8,547,099 B2). With this NMR system, the tilt of the rotation axis (z′-axis) with respect to the probehead and the magnet system is kept constant, and the direction of the static magnetic field is rotated by generating a field B1 with an additional electromagnetic coil. The additional electromagnetic coil is arranged around the sample, so that the angle between the z′-axis and the direction of the linear combination of B0 and B1 fields, corresponds to the magic angle.
Although this method enables the fast and precise adjustment of the angle, introducing an additional electromagnetic coil into the magnetic system produces a disadvantage in that the diameter of the probehead must be reduced in the area around the sample. Furthermore, the adjustment range is limited to very small angle corrections in the range of less than 0.05° (see reference [8]) due to the dissipation in the additional coil generated in operation.
Reducing the outer diameter of a probehead that has an internal adjustment mechanism leads to various disadvantages:                The space available for the technical realization of the sample rotation (e.g., air bearings, drive, gas pipes, etc.) decreases. This increases the complexity and generates additional costs.        The space available for the RF fields within an electrical shield decreases, leading to reduced RF performance of the probeheads. This is expressed in reduced signal-to-noise ratio of the measurement, as well as increased performance requirements and accompanying higher dissipation needed to achieve the pulse durations.        The available space for gas pipes, the RF network, optional heat exchangers, pump lines, and adjustable RF elements is reduced. This can have a negative influence on the performance of various components of the probehead.        
For this reason, the reduction of the probehead diameter should be as small as possible, at least in the central area.
Two types of magnet systems exist on the market: standard bore (SB) magnet systems and wide bore (WB) magnet systems. These magnet systems have bore holes with a diameter DB of around DB=40 mm+/−2 mm for SB systems and DB=73 mm+/−2 mm for WB systems.
For SB magnet systems, diameter reductions of more than 10 mm are typically unacceptable for the performance of samples. For many applications, a 5 mm diameter reduction is already harmful. As a sufficiently powerful additional electromagnetic coil for angle correction cannot be produced in this reduced volume, a purely electrical adjustment of the magic angle for SB probeheads, particularly for ultra high field NMR with magnetic field strengths B0>20 T, is not realistic.
To provide context for better understanding the improvements associated with the present invention, a typical NMR-MAS probehead known from the prior art will first be described
In the prior art, MAS probeheads generally include an adjustment mechanism, which enables the angle θ between the rotation axis of the sample along z and the static magnetic field B0 along z to be precisely adjusted within the range θtarget−α1≤θ≤θtarget+α1. Such an adjustment mechanism integrated into the probehead is described as an “internal” or “integrated” mechanism. In this mechanism, in general, the sample, the stator, containing the bearing and the drive of the rotor and the RF coils are moving. This movement is induced by hoists, spindles and gearwheels, levers with linear movements or similar mechanisms, and predominantly takes the form of rotation movement, although rotation movements combined with linear movements are also common. Adjustment mechanisms provided with manual and motorized adjustment are known, particularly provided with electromotive adjustment. With many prior art probeheads, particularly those that can be used in SB magnet systems, the angle adjustment can be carried out over a very large range, which enhances the ease with which the samples can be exchanged.
It is known that particularly when the sample temperature changes, these adjustment mechanisms are often not sufficiently precise in the context of demanding NMR measurements. This particularly applies to proton spectroscopy and STMAS, where angle errors in the range of just a few thousandths of a degree can lead to noticeable line broadenings in the measured spectra. An additional problem occurs with probeheads with cryogenically cooled detection coils. In general, the coils are separated from the samples by an insulation vacuum. To this end, there is usually at least one wall of a Dewar between the detection coils and the sample. It is mechanically very onerous to integrate tilting of samples, the dewars, and the RF coils within a probehead, (although this was in fact the method disclosed in U.S. Pat. No. 7,282,919 B2 in the context of WB probeheads.
In the prior art, the following method is used to adjust the angle of θ between the rotation axis of the sample and the magnetic field direction: in general, a sample (e.g. powdered potassium bromide) with the greatest possible dependency of the line width on the adjusted angle is measured using the NMR probehead, and the line widths of the central line, and rotation side bands and/or the height of the lines and/or the ratio of amplitude/width between various lines are evaluated. Alternatively, an evaluation can be carried out directly on the time domain signal. In this method, the angle is adjusted by activation of an adjustment mechanism, which tilts the stator relative to the z-axis, around a tilt axis. Since the stator specifies the rotation axis, the rotation axis is rotated against the z-axis around the tilt axis. The tilt axis is mounted against the probehead, which does not change its position in the magnet system.
In general, prior art probeheads have a contour which consists of a cylindrical pipe with a substantially constant outer diameter.
Often, these probeheads also have options for centering and positioning against the magnet system (generally in the area of the upper and lower end of the part of the probehead inserted in the cavity), in order to achieve the clearest possible defined positioning, when the probehead is installed in the magnet system. In this manner, the probeheads can use the maximum available volume of an ordinarily circular cylindrical cavity of the magnet system as efficiently as possible.